Centrifugal cleaners have been known for decades. In a typical use of a centrifugal cleaner it is desirable to remove as many contaminants (rejects, debris) as possible while removing as little desirable material (accepts) as possible, i.e. to have the highest practical cleaning efficiency. Many different to structures and implementation schemes have been designed to accomplish this desirable end result, however conventional cleaners still are not as effective as desired for many applications. For example, in the pulp and paper industry the consistency of the fiber suspension to be treated tends to vary for a number of reasons, and there is a continuing desire to use higher consistency suspensions to decrease the amount of water used for diluting the pulp for centrifugal cleaning. It has, however, been found that the cleaning efficiency of conventional centrifugal cleaners is extremely sensitive to consistency, and if the consistency of the fiber suspension increases the efficiency of the cleaner drops dramatically. This is believed due at least in part to the fact that the cleaner recognizes pulp flocs (which naturally have a higher specific gravity than individual fibers) as knots or stickies, and therefore treats them as rejects. The amount and/or size of pulp flocs tends to increase with increasing suspension consistency.
In pulp and paper making, environmental demands necessitate recycling of paper. The paper, depending on its origin, contains more or less fillers, ink, etc. i.e. matter that should be removed as efficiently as possible. Centrifugal cleaners have been used for removing this undesired matter with some success. However, it has been found that ink particles, especially originating from laser printers, are extremely difficult to remove, but as the demand for offices to recycle wastepaper grows the amount of recycled paper containing laser ink increases rapidly.
According to the present invention, a number of improvements are provided to conventional centrifugal cleaners which remarkably improve their efficiency and/or versatility, which improvements can be incorporated in new cleaners or retrofit into existing cleaners.
Virtually all centrifugal cleaners have a generally hollow main body with a side wall having a cylindrical body portion and a generally decreasing conical body portion tapering from the top toward the bottom, a tangential inlet nozzle in the side wall near the body top in the cylindrical body portion for introducing fluid material to be cleaned, a top outlet nozzle (commonly known as a "vortex finder ") extending downwardly into the body through the top and centrally located in the body, the bottom of the top nozzle extending below the tangential inlet nozzle, and a bottom outlet nozzle disposed generally concentrically with the top outlet nozzle, and spaced from the tangential inlet nozzle. The improvements according to the invention relate to the configuration of one or all of the tangential inlet nozzle, the cylindrical body portion and the vortex finder.
A typical tangential inlet nozzle is of conically tapering configuration in the fluid flow direction. For example see U.S. Pat. Nos. 2,756,878, 2,793,748, 2,816,658, 3,306,461, 3,349,548 and 3,807,42. It has been found according to the present invention that a tapering configuration is far from ideal, causing minimal turbulence, which means in practice that even small variations in the consistency of the fluid have a dramatic effect on the efficiency of the centrifugal cleaner. Existing centrifugal cleaners have high removal efficiencies at 0.5-0.6% feed consistency, but efficiency drops significantly as consistency increases. A centrifugal cleaner that efficiently removes unwanted particles (rejects) from pulp at consistencies of 1.0% or higher has a number of advantages, including allowing utilization of a less costly deinking system, and requiring only about one-half of the water consumption (or treatment) of a conventional low consistency (0.5-0.6%) system.
The increase in the consistency of a fiber suspension means in practice that the fibers are closer to each other and, therefore, form flocs i.e. groups of fibers, more easily. Since the fiber flocs decrease the efficiency of the cleaner the formation of flocs should be prevented. According to the present invention, an inlet nozzle having turbulence generating capabilities is provided. A turbulence generator prevents an increase in suspension consistency from decreasing the efficiency of the cleaner by preventing the flocs from forming in the nozzle and/or by breaking up already formed flocs.
According to one aspect of the present invention a centrifugal cleaner for fiber suspensions having fiber flocs therein is provided. The cleaner comprises the following elements: A generally hollow main body having a top and a bottom and a side wall having at least a portion thereof with a generally decreasing conical taper from the top toward the bottom thereof, and having a tangential inlet in the side wall near the body top for introducing fiber suspension to be cleaned. [In the specification and claims the terms "top" and "bottom" are used for reference purposes only, and do not require any particular orientation. While usually the "top" is directly vertically above the "bottom", the "top" and "bottom" may be horizontally in line or the "bottom" above the "top", or a wide variety of other orientations may be provided.]A vortex finder located in the body top. A bottom outlet nozzle located at the bottom of the main body, substantially concentric with the vortex finder. And, a turbulence generator disposed in the tangential inlet for generating sufficient turbulence so as to break up fiber flocs in introduced suspension and prevent reformation of the flocs before the suspension enters the hollow main body, so as to enhance cleaning efficiency of the cleaner, increase the consistency of fiber suspension which the cleaner can effectively handle, and/or minimize the sensitivity of the cleaner cleaning efficiency to consistency changes in the fiber suspension compared to the same cleaner but not including the turbulence generator.
The turbulence generator preferably comprises an abrupt cross-sectional area reduction portion in the tangential inlet; e.g. the turbulence generator portion has a cross-sectional area of about 0.1-0.3 times as large as the cross-sectional area of the inlet. Where the inlet is substantially circular in cross-section having a first diameter, the turbulence generator reduced cross-sectional area portion has a second diameter which is about 0.35-0.55 (preferably 0.4-0.5, e.g. 0.46) times as large as the first diameter.
Alternatively the turbulence generator may comprise a plurality of surface manifestations in the inlet causing a fluctuating cross-sectional area within the inlet from near the beginning of the inlet to the hollow main body. The surface manifestations may comprise a plurality of circumferential grooves which are polygonal in cross-section, or a spiral rib having a height of about 15-25% of the diameter of the inlet, or comparable surface manifestations. Alternatively the turbulence generator may comprise a zig-zag configuration of the inlet which causes the fiber suspension to flow in a tortious path.
The invention also relates to a method of reconstructing a conventional centrifugal cleaner, that is retrofitting the conventional cleaner so as to achieve the advantages according to the invention. The method is practiced by the step of inserting into the inlet a turbulence generator and positioning the turbulence generator within the inlet. For example this may be accomplished by inserting into the inlet a turbulence generator having an exterior cross-sectional area and configuration corresponding to the first cross-sectional area and configuration and an interior second cross-sectional area about 0.1-0.3 times the first cross-sectional area, and having a second length significantly less than the first length; and positioning the turbulence generator in the inlet so that there is an abrupt cross-sectional area decrease in the pathway of fibrous suspension flowing into the inlet and to the body. This also may be effectively, or alternatively, practiced by inserting into the inlet a turbulence generator having an exterior cross-sectional area and configuration corresponding to the first cross-sectional area and configuration, and an interior passage for generating sufficient turbulence so as to break up fiber flocs in introduced fiber suspension and prevent reformation of the flocs before the suspension enters the hollow main body, so as to enhance cleaning efficiency, increase the consistency of fiber suspensions the cleaner can effectively handle, and/or mininimize the sensitivity of the cleaner to consistency changes in the fiber suspension compared to the same cleaner but not including the turbulence generator.
According to another aspect of the present invention, cleaning efficiency is enhanced even further by providing a particular ratio of the vortex finder diameter to the cleaner body diameter, and by providing a particular length of the vortex finder into the cleaner body, a length significantly longer than is typically utilized. Surprisingly a longer vortex finder does not necessarily result in enhanced short circuit prevention of introduced pulp to the accepts outlet, but it does have a significant positive affect on debris removal efficiency. The particular construction of the vortex finder according to the present invention can be used in combination with a turbulence generator as set forth above, or independently.
According to this aspect of the present invention, a centrifugal cleaner for fiber suspensions is provided which comprises the following elements: A generally hollow main body having a top and a bottom and a side wall having at least a portion thereof with a generally decreasing conical taper from the top toward the bottom thereof, and having a tangential inlet in the side wall near the body top for introducing fiber suspension to be cleaned. A vortex finder located in the body top. A bottom outlet nozzle located at the bottom of the main body, substantially concentric with the vortex finder. Wherein the vortex finder has a first diameter and the hollow body has a second diameter at a portion thereof surrounding the vortex finder. And, wherein the first diameter is about 0.25-0.4 times the second diameter.
The vortex finder extends into the hollow body a first length from the top, the first length to first diameter ratio being about 2.5-3.5/1. The first diameter is most preferably about 0.3-0.5 times the second diameter, while the first length the first diameter ratio is preferably about 2.5-3.1/1.
Also, it has been found that cleaning efficiency is enhanced when, in conjunction with the longer vortex finder described above any cylindrical portion of the generally hollow main body is minimized or eliminated. For example excellent efficiency is obtained when the side wall from the tangential inlet toward the bottom is substantially completely defined by the conically tapered portion, and wherein the conically tapered portion has an angle of taper of about 2.degree.-6.degree..
The advantages of this aspect of the present invention may also be achieved by reconstructing (retrofitting) existing cleaners. For example where a cleaner body has a first diameter and the vortex finder has a first length from the top of the cleaner into the body, there may be the step of replacing the vortex finder with a replacement vortex finder having a second length greater than the first length, and a second diameter, the ratio of the second length to the second diameter being about 2.5-3.1/1. The replacing step may also or alternatively be practiced by replacing the vortex finder with a replacement vortex finder having a second length greater than the first length and a second diameter, the second diameter being about 0.3-0.35 times the first diameter.
It has also been found according to the present invention that a smaller retention time in the centrifugal cleaner for the pulp actually results in better cleaning efficiency. While the retention time differs significantly depending upon the size of conventional cleaners, retention times typically range from about 0.55-1.95 seconds. In general smaller diameter cleaners have shorter retention times and larger cleaners have longer retention times. Once a particle is moved to the cleaner wall or outside diameter it can be assumed to be removed. Accepts are skimmed off near the core and since the debris particles have been forced to the cleaner walls the core pulp is clean. In the worst case scenario a particle has to migrate from the central "air" core to the cleaner wall, this distance roughly being equal to the cleaner's radius. In a conventional three inch cleaner this distance is 1.5 inches while in a 12 inch cleaner the distance is six inches. Assuming there is a reasonable settling rate of particles at three inches per second, a three inch cleaner needs 0.5 seconds to remove the particle while a 12 inch cleaner needs two seconds. Providing no cylindrical portion of the cleaner body, but merely the conical taper, reduces the cleaner volume and thus the retention time, with an optimum retention time of less than about 0.5 seconds being optimum for a three inch cleaner.
It is the primary object of the present invention to provide a centrifugal cleaner having enhanced cleaning efficiency, the ability to efficiently clean fiber suspensions of significantly higher consistency than in the prior art, and/or to provide a cleaner less susceptible or sensitive to consistency changes in the fiber suspension. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.